Marine sediments in four piston cores collected from the northwest African continental margin during Biogeochemical Ocean Flux Study cruise 53 (RRS Charles Darwin) were comprehensively studied using a wide range of physical sediment parameters and geochemical techniques. The objective of the study was to examine the Late Quaternary palaeoceanography of the Northwest African continental margin and determine changes in the climate of the north and central African continent over the past 600 kyr. In addition, the core sites formed a transect running SE-NW, enabling the spatial analysis of upwelling intensity and wind vigour proxies. Chronostratigraphy was determined by correlating planktic (<I>G.inflata</I>) and benthic (<I>C.wuellerstorfi</I>) oxygen isotope (δ<SUP>18</SUP>O) records with the SPECMAP records of Imbrie <I>et al</I>. (1984) and Martinson <I>et al</I>. (1987). Sediment accumulation rates are greater during glacial stages and generally decrease with distance from the continent. North and central African continental aridity variations are indicated by the Al flux records. The calcium carbonate records also approximate aridity fluctuations due to dilution by lithogenic dust supplied by the Harmattan and northeast Trade wind systems. These changes operate dominantly at the 41 kyr (obliquity) Milankovitch frequency cycle and display highest Saharan dust input and aridity during glacial stages. Zr/Rb, Ti/Al and Lithogenic Component Grain Size profiles indicate an increase in grain size, and hence wind vigour, during glacial stages and the stage 5 interstadials, and hence display a strong 23 kyr (precessional) component of forcing as well as at the 100 kyr (eccentricity) frequency band. The most distal core site in the transect is strongly influenced by the kaolinite-rich Harmattan wind supply, while the three more proximal cores are affected by dust input from the northeast Trade winds. A high-resolution calcium carbonate profile of another core in the transect confirms the presence of high-amplitude, high-frequency climatic perturbations (aridity changes) with mid-termination setbacks similar to the Younger Dryas event which is seen in many north Atlantic climate records. This high latitude climatic deterioration also influenced north African climate, resulting in increased aridity and wind vigour at approximately 11,000 years BP. Similar climatic conditions are noted over the last 7,000 years. A sawtooth pattern in the high-resolution calcium carbonate and Bulk Sediment Grain Size records implies that rapid increases in continental aridity were followed by gradual increases in humidity, and that the insolation-induced African climate system is decoupled from global (ice-volume) climate change operating at a glacial/interglacial timescale.

Our knowledge of the global carbon budget is at present insufficient to predict reliably the possible consequences of rising CO<SUB>2</SUB> levels on the climate and on ecosystems. Attempts to close the global carbon budget have indicated that a major sink for CO<SUB>2</SUB> may exist in the boreal forests of the northern hemisphere, (Tans <i>et al</i>., 1990). This thesis reports on field experiments which were undertaken to investigate the turbulent exchange of mass, energy and momentum in a northern hemisphere coniferous forest, with special attention to the exchange of carbon dioxide. The eddy covariance technique was used to measure mass, energy and momentum fluxes above and within a Sitka spruce, (<i>Picea sitchensis</i> (Bong.) Carr.), plantation in South West Scotland. The forest had a mean height, <i>h</i>, of 15 m, was densely planted and unthinned. Measurements were made at heights of 1.2<i>h</i>, 0.7<i>h</i> and 0.1<i>h</i>, where the heights within the canopy corresponded to the lower canopy crown and the dead branch trunkspace, respectively. The upper canopy was found to be efficient at attenuating momentum, with momentum fluxes at 0.7<i>h</i> being only 2 - 3% of those at 1.2<i>h</i>. Spectral slopes of velocity and scalar quantities were considerably steeper than expected from Similarity theory indicating a more rapid dissipation of larger scale motions than above the canopy. Evidence was found to support the existence of large, intermittent and energetic structures dominating the turbulent exchange in the forest. Eularian integral time scales for streamwise (<i>L</i><SUB>u</SUB>) and vertical (<i>L</i><SUB>w</SUB>) velocities were calculated at 1.2<i>h</i> as ~ <i>h</i> and ~ 0.2<i>h</i>, respectively. A momentum quadrant analysis indicated that ~ 50% of momentum flux, 21 - 47% of CO_2 flux and 23 - 44% of sensible heat flux at 1.2<i>h</i> was accounted for by events with instantaneous momentum fluxes greater than four times the mean value, which occurred only 5 - 12% of the time.

This thesis is concerned with the influence of the weather on radio signals on surface-to-surface paths, looking particularly at the effects of fronts. The radio-meteorology of anticyclones is well understood [COST 1991], but that of fronts is less so, despite the knowledge that fronts can sometimes cause very severe radio interference through anomalous signal propagation (anaprop). A statistical analysis is made of two years signal data from seven paths in the UK and to the Netherlands. By classifying weather conditions into 24 different types, the effects of different types of weather as causes of anaprop have been determined. The results confirm the belief [Bye 1988a] that the majority of anaprop occurs under anticyclonic conditions, and that fronts are a relatively insignificant cause of anaprop. The results for the different weather and path types are presented in a set of 'interference data sheets', allowing rapid comparison of the effects of different weather conditions on signals for land and sea paths. This analysis is one of only two that examine the effects of different weather conditions on signals, and considers a far greater amount of both signal and weather data than the other study [Spillard 1991]. To examine how fronts can cause anaprop, existing meteorological conceptual models [e.g. Browning 1985] are adapted to show where super-refractive layers occur. The models examine ana- and kata-fronts (both warm and cold), as well as warm and cold occlusions. For each type of front, qualitative predictions of the likelihood of anaprop are given. The conceptual models are verified in two ways. Using dropsonde data from the FRONTS'87 project, three fronts are examined at resolutions far higher than can be obtained from routine observations. Super-refractive layers are found where the conceptual models predict them, and it is possible to make estimates of the location and strength of these layers.

A Lagrangian column model, featuring a detailed description of vertical mix­ing, has been developed to simulate the three-dimensional structure in ozone and nitrogen oxides <i>(NOx) </i>concentrations in the boundary-layer, within and down­wind of high-latitude urban areas. The short time-scale photochemical processes of ozone and <i>NOx, </i>as well as emissions and deposition to the ground, are simu­lated. This has been used to follow a range of one-dimensional trajectories, over a distance of 100km and a travel time in the order of 10⁴s, through a simulated city under a variety of meteorological and pollutant emission regimes representing seasonal and diurnal extremes. An assessment of the extent of ozone destruction occurring, the rate of recovery of surface ozone concentrations downwind of the city and the influence of meteorological parameters on the ozone concentration, has been provided using the model. The model, with a spatial resolution of 1x1km, is also applied over a l00xl00km domain containing a simulation of the <i>NOx </i>emission field from the city of Edin­burgh, UK (at latitude 56⁰N) to simulate the two-dimensional city-scale processes of pollutants. Results are presented, using averaged wind-flow frequencies and appropriate stability conditions, to show the extent of the depletion of ozone by the city. The long-term average spatial patterns in the surface ozone and <i>NOx </i>concentrations over the model domain are reproduced quantitatively and are compared with observations from a series of monitoring sites on a north-south trajectory through the city and into rural areas. The model shows the average surface ozone concentrations in the urban area to be lower than the surrounding rural areas by typically 50% and that the areas experiencing a 20% ozone de­pletion over background values are generally restricted to within the boundaries of the urban area. From vertical profiles of ozone concentrations, the depletion of ozone to less than 50% of the rural surface values generally extends not more than 20m vertically above the urban area.

Turbulence data were collected above and within the canopy of a black spruce forest in central Canada (in the southern study area of BOREAS). Streamwise (<I>u</I>), transverse (<I>v</I>) and vertical (<I>w</I>) wind speed were measured with 3-component sonic anemometers. Scalar transport within the canopy was studied using the heat fluxes from the sonic anemometers, and water vapour fluxes from two open-path infra-red gas analysers. As a result of instrumental errors, no CO<SUB>2</SUB> fluxes were obtained. Tree height (<I>h</I>) is approximately 10 m, and leaf area index is about 5. The narrow tree crowns produce a very open canopy. A zero plane displacement of 0.45 <I>h</I> was obtained from the mean momentum flux profile. This is lower than typical values, as was the attenuation coefficient, <I>a</I>, calculated from the mean wind speed profile, of 1.9. These probably result from sparse foliage in the upper canopy, allowing relatively unrestricted air movement. The roughness length was found to equal 0.11 <I>h. </I>Profiles of mean velocity statistics were constructed to describe the turbulence regime within the forest canopy. Turbulence intensities were found to be high in the lower levels of the canopy. Above the canopy, skewness and kurtosis of the three velocity components had values close to those for a Gaussian distribution. In contrast, within the canopy, skewnesses were negative for <I>w, </I>and positive for <I>u. </I>Kurtosis values were high (up to 12 close to the ground), and integral Eulerian length scales revealed eddies with dimensions of the order of canopy height at the top of the canopy. These indicate that the air flow within the canopy is characterised by intermittent gusts of air penetrating from above the canopy. The turbulence profiles, when scaled by <I>h</I> or fraction velocity, agree well with profiles from a wide range of other canopies. Non-dimensionalised variances of <I>w, </I>temperature and water vapour fluctuations, measured above the canopy, were found to vary with stability in accordance with Monin-Obukhov similarity theory. Quadrant analysis showed the transport of momentum and scalars to be intermittent, with 50% of momentum transport occurring in 3 - 8% of the time. Instantaneous events six times larger than the mean scalar flux transported 20 - 50% of the heat flux in 3 - 4% of the time, and 50 - 80% of the water vapour flux in 12 - 20% of the time. Transport mechanisms were similar for all three entities, being dominated by ejections (updraughts) above the canopy, and gusts (downdraughts) within the canopy.

On September 12, 1991 the Upper Atmosphere Research Satellite (UARS) was launched. It carried a number of instruments one of which was the Microwave Limb Sounder (MLS) (Barath et al., 1993). MLS obtains near-global simultaneous measurements of several atmospheric trace species at heights throughout the middle atmosphere. One of the species measured is water vapour. This thesis describes the zonal-mean climatology of the MLS water vapour measurements in the stratosphere (˜16 to 50 km) and compares it to previous satellite measurements of <I>H<SUB>2</SUB>O </I>from the Limb Infrared Monitor of the Stratosphere (LIMS) and the Stratospheric and Aerosol Gas Experiment II (SAGE II) (e.g. Russell et al., 1984, Chiou et al., 1993)). Some of main features of the distribution in MLS data, the increase in mixing ratio with height, the presence of a tropical minimum in the lower stratosphere, are consistent with these previous results. The extensive dehydration that occurs in the Antarctic polar vortex during winter is comprehensively observed for the first time in UARS data. Water vapour measurements in the arctic winter vortex do not show any significant dehydration. Temporal and spatial variations are also discussed and it is shown that the zonal-mean variability in the lower tropical stratosphere is primarily annual and is modulated by the quasi-biennial oscillation (QBO) in zonal-mean winds. This modulation means that the time between maxima and minima is either shorter or longer than one year according to the phase of the QBO. At higher latitudes, the variability in the lower stratosphere is marked by periods of descent (and dehydration) in the winter polar vortices. In the upper stratosphere, there is a clear semi-annual oscillation in tropical mixing ratios which compares well to the variability in zonal-mean winds in these regions. The location of the maximum amplitude of the H<SUB>2</SUB>O oscillation moves seasonally between hemispheres. At higher latitudes the variability is annual and is marked by periods of stronger winter descent than in the lower stratosphere, consistent with calculations of high-latitude descent rates (e.g. Schoeberl et al., 1992). The annual variations are complicated by the fact that air is drier in the mesosphere than in the stratosphere so that descent from aloft first brings wet air (and thus maxima) and then dry air (and minima).

This thesis is concerned with the development, and subsequent use of a new trajectory mapping technique to produce synoptic analyses of water vapour in the middle atmosphere. The technique has been applied to water vapour measurements in the middle atmosphere made by the Microwave Limb Sounder instrument (MLS) aboard the Upper Atmosphere Research Satellite (UARS). These measurements cover the time period early October 1991 until mid-April 1993. The technique assumes that 1) the water vapour data used to initialise the trajectories are conserved and 2) the trajectories are adiabatic for the length of the advection. The technique uses a simple trajectory model and numerical wind fields interpolated to isentropic surfaces to produce equal-area gridded analyses. Trajectories are initialised using MLS water vapour data and have their value altered as MLS measurements are assimilated into the trajectory scheme at their time and position of measurement. In this manner the evolution of the water vapour field is constrained as much by the MLS water vapour measurements as by the wind fields, in contrast to pure trajectory techniques which are heavily sensitive to the quality of the meteorological analyses. The synoptic analyses produced by the technique are shown to greatly enhance the measurements of the MLS by increasing the spatial resolution and are more accurate than analyses produced by gridding the daily data and approximating this to be a synoptic map. Analyses have been used in two case studies of atmospheric phenomena. The first has been to quantify, over an annual timescale, the irreversible transport of tropical air into northern and southern hemisphere mid-latitudes. Version 3 data were used. It is found that during 1992 roughly fifty per cent more tropical air is exchanged into northern mid-latitudes than into southern mid-latitudes. The exchange is associated with breaking planetary waves episodically disturbing the poleward edge of the tropical reservoir. The second case study involved looking at the temporal evolution of Arctic water vapour measured during the northern hemisphere winters of 1991-92 and 1992-93.

The earth is bathed in an ever changing magnetic environment due to fluctuations in the solar wind. The external field induces currents within the earth which cause a secondary internal field. The ratio of internal to external parts of the magnetic potential is known as the response and may be derived from measurements of the field at the surface. The response of the earth is dependent on the spatial form of the field and distribution of conductivity within the earth. The analysis of numerous geomagnetic storms, large disturbances in the field surrounding the whole earth, is presented with the aim of determining a reliable response function which enables an estimate to be made of the average conductivity of the upper mantle. The compilation of a database of geomagnetic storms was a major part of the work. All the suitable storm events were selected between 1957 and 1982 to give 44 storms. The entire set of hourly values were checked for errors and corrections made where necessary. Where data were missing their values were interpolated using information from nearby observatories. The lower the frequency of external magnetic variations the deeper the penetration into the earth. The frequency content of geomagnetic storms allows depths approaching 1500km to be investigated. The observations of magnetic field were Fourier transformed and attention focussed on the Fourier coefficients of the lowest frequencies, 0.03 to 1 cycle per day. From Spherical Harmonic Analysis in the frequency domain it was found that a pure <i>P</i><SUB>1<SUP>o</SUB></SUP> spherical harmonic model is acceptable for the spatial form of the field at the frequencies of most interest. Thus the source is assumed to be a simple ring current in common with most of the previous research. The Fourier coefficients of the X and Z magnetic components were then fitted to the appropriate <i>P</i><SUB>1<SUP>o</SUB></SUP> model which allows the separate internal and external parts to be evaluated. A robust method, to reduce the influence of anomalous values, was used for determining the optimum fit to the Fourier coefficients. The technique was assessed by examining the distribution of residuals.

No description available.

The Upper Atmosphere Research Satellite (UARS) was launched on 12<SUP>th</SUP> September 1991 carrying a payload to measure the chemistry, dynamics and energy balance of the middle atmosphere. The Microwave Limb Sounder (MLS) onboard UARS makes measurements of atmospheric thermal emission at millimetre wavelengths which are used to infer molecular abundances throughout the stratosphere and mesosphere. Measurements made at 183.3 GHz are used to retrieve concentrations of water vapour (H<SUB>2</SUB>O). Measurements of water vapour on a global scale are of particular importance to the study of stratospheric photochemistry, radiation budget and dynamics. MLS provides approximately 19 months of near continuous water vapour measurements on a global scale. A characterisation and error analysis, based on the formalism of <I>Rodgers, [1990]</I>, is performed for the UARS MLS 183 GHz H<SUB>2</SUB>O retrievals. The useful vertical range for scientific studies with MLS Version 3 H<SUB>2</SUB>O retrievals is found to be 22-0.2 hPa at high latitudes and 46-0.2 hPa elsewhere. For this vertical range, the estimated vertical resolution of the retrievals is 5-6 km. In the error analysis, estimates of the contributions of both random errors, including the smoothing error, and systematic uncertainties to the overall error in retrieved H<SUB>2</SUB>O are produced. It is found that the retrievals at 46 hPa and 22 hPa contain an increased contribution from the <I>a priori</I> information at high latitudes during the polar winter which is due to a corresponding loss of information from the radiance measurements. This is linked to the use of an <I>opacity criterion</I> which does not permit radiance measurements which arise from optically thick limb paths to be included in the retrieval. A new opacity criterion is developed in an attempt to improve the retrieval at 46 hPa and 22 hPa. Test retrievals show that it is possible to reduce the contribution from the <I>a priori</I> information to an acceptable level, but at 22 hPa a systematic increase in the H<SUB>2</SUB>O mixing ratio occurs which requires further investigation.